1. Field of the Invention
The present invention relates to a plasma etching apparatus for processing a semiconductor substrate, such as a semiconductor wafer, and to a plasma etching method using the plasma etching apparatus.
2. Description of the Related Art
Conventionally, a plasma etching apparatus utilizing reactive plasma has been used to process a semiconductor substrate, such as a semiconductor wafer, in a manufacturing process of a semiconductor device.
As an example of the plasma etching, etching for forming a polysilicon (poly-Si) gate electrode of a metal oxide semiconductor (MOS) transistor (hereinafter referred to as gate etching) is described below with reference to FIGS. 7A to 7C. As illustrated in FIG. 7A, a specimen 1 before etching is composed of a silicon (Si) substrate 2, a silicon dioxide (SiO2) film 3, a polysilicon film 4, an antireflection coating (also referred to as bottom anti-reflective coating: BARC) 5, and a photoresist mask 6, from the bottom. The gate etching involves a BARC etching process and a polysilicon etching process. In the gate etching, the polysilicon film 4 in an area not covered with the photoresist mask 6 or the antireflection coating 5 is removed by exposing the specimen 1 to reactive plasma.
In the BARC etching process, the antireflection coating 5 in an area not covered with the photoresist mask 6 is removed by exposing the specimen 1 to reactive plasma. After the BARC etching process, an antireflection coating 5′ remains under the photoresist mask 6, as illustrated in FIG. 7B. In the polysilicon etching process, as in the BARC etching process, the polysilicon film 4 in an area not covered with the photoresist mask 6 and the antireflection coating 5′ is removed. After the polysilicon etching process, as illustrated in FIG. 7C, a gate electrode 7 is formed on the silicon dioxide film 3 under the photoresist mask 6 and the antireflection coating 5′.
The gate width 9 of the gate electrode 7 has a large influence on the performance of an electronic device and is therefore strictly controlled as the most critical dimension (CD). A value calculated by subtracting the width of a mask before etching from the width of a film after etching is referred to as “CD shift.” The CD shift is an important indicator to determine the quality of etching. Thus, a target CD shift is defined before etching. In the BARC etching process, the CD shift is calculated by subtracting the width 8 of the photoresist mask 6 from the width 8′ of the antireflection coating 5. Similarly, in the polysilicon etching process, the CD shift is calculated by subtracting the width 8′ of the antireflection coating 5 from the gate width 9. The CD shift of the gate etching is calculated by subtracting the width 8 of the photoresist mask 6 from the gate width 9.
A conventional plasma etching apparatus for the gate etching is described below with reference to FIG. 8. The conventional plasma etching apparatus includes a processing chamber 26 composed of a processing chamber lid 22 and a showerhead plate 24 on a processing chamber wall 20, and a specimen holder 28 in the processing chamber 26. A specimen 1 is placed on the specimen holder 28. A processing gas 36 is introduced into a space 32 between the processing chamber lid 22 and the showerhead plate 24 through an inlet 30 provided in the upper portion of the processing chamber wall 20. The processing gas 36 is then introduced into the processing chamber 26 through a lot of gas inlet holes 34 provided in the showerhead plate 24 and generates plasma 38. A plasma etching process is performed by exposing the specimen 1 to the plasma 38. The processing gas 36 and volatile substances produced during plasma etching are eliminated from an exhaust port 40. The exhaust port 40 is connected to a vacuum pump (not shown). The internal pressure of the processing chamber 26 is reduced to about 0.5 to about 1 Pa with the vacuum pump.
While the gate etching is performed with such a plasma etching apparatus, an increase in the size of the specimen 1 makes it difficult to achieve in-plane uniformity of the etch rate over a wider area of the specimen 1, the CD shift of the gate etching, and the shape of the gate electrode 7. Furthermore, recent finer design of a semiconductor device requires stricter control of the CD shift.
Deposition of a reaction product on the sidewall of a gate electrode is described below as a factor having an influence on the CD shift. Conventionally, a gas mixture, for example, of chlorine (Cl2), hydrogen bromide (HBr), and oxygen (O2) has been used in the gate etching. These gases are present in a plasma state during etching, serving as etchants for the polysilicon film 4. During etching, chlorine (Cl), hydrogen (H), bromine (Br), and oxygen (O) ions or radicals generated by the dissociation of chlorine, hydrogen bromide, and oxygen molecules in the processing gas 36 react with silicon from the polysilicon film 4 to produce reaction products. The ions are attracted to the specimen 1 owing to a high-frequency wave applied to the specimen 1 and perform anisotropic etching, thus providing a gate having a vertically well-defined desired shape. Although volatile substances in the resulting reaction products are eliminated from the exhaust port 40, nonvolatile substances are deposited on the polysilicon film 4 or the photoresist mask 6. Among the nonvolatile substances, nonvolatile reaction products deposited on the sidewall of the gate electrode 7 act as a protective film of the sidewall against etching by an etchant radical, such as chlorine. Thus, when a small amount of nonvolatile reaction products are deposited on the sidewall of the gate electrode 7, isotropic etching due to radicals often proceeds on the sidewall of the gate electrode 7, decreasing the gate width 9 after etching. On the other hand, when a large amount of nonvolatile reaction products are deposited on the sidewall of the gate electrode 7, the gate width 9 after etching often increases because they act as an etching mask.
Accordingly, the concentration of reaction products has a large influence on the gate width 9. A nonuniform distribution of reaction products in the vicinity of the surface of the specimen 1 may cause in-plane nonuniformity of the CD shift of the specimen 1. For example, while a central portion of the specimen 1 has silicon to be etched around the central portion, the perimeter of the specimen 1 has no silicon to be etched outside the perimeter. Thus, even when the etch rate is uniform on surface of the specimen 1, the concentration of reaction products containing silicon derived from the polysilicon film 4 is high in the central portion and low in an outer portion. This can also cause in-plane nonuniformity of the CD shift.
Furthermore, in-plane nonuniformity of an etchant, such as a chlorine radical or ion or a bromine radical or ion, in the vicinity of the surface of the specimen can cause in-plane nonuniformity of the etch rate, which can in turn cause in-plane nonuniformity of the CD shift. In addition, when a silicon-based reaction product produced in etching using chlorine or bromine is chemically combined with oxygen, the product is more easily deposited on the sidewall of the gate electrode 7. Thus, in-plane nonuniformity of the oxygen content may cause in-plane nonuniformity of the CD shift. Similarly, when a processing gas containing carbon, such as carbon tetrafluoride (CF4), is used, the resulting carbonaceous reaction product is more easily deposited on the sidewall of the gate electrode 7. Thus, in-plane nonuniformity of the concentration of the carbonaceous reaction product may cause in-plane nonuniformity of the CD shift.
As described above, nonuniform distribution of a reaction product or an etchant on the surface of the specimen 1 may cause in-plane nonuniformity of the CD shift. To improve in-plane uniformity of the CD shift, the inventors have focused attention on the importance of controlling the distribution of a reaction product in the vicinity of the surface of a specimen and the composition of a processing gas. Thus, the inventors proposed a technique in which gases having different compositions are introduced from a plurality of gas inlets (see, for example, Japanese Unexamined Patent Application Publication No. 2005-056914). However, this technique does not refer to individual control in a plurality of etching steps constituting the etching process. Furthermore, this technique is insufficient to reduce nonuniformity of the CD shift caused by variations in the surface density.
The amount of reaction product deposited on the sidewall of the polysilicon film 4 (that is, the side wall of the gate electrode 7 during etching), which is one of factors having an influence on the gate width 9, varies with the temperature of the specimen 1 during etching. This is because the sticking coefficient of a reaction product to the sidewall of the gate electrode 7 increases with decreasing temperature. Thus, the temperature distribution on the surface of the specimen 1 has an influence on the in-plane distribution of the CD shift. Japanese Patent Application No. 2005-028804 discloses a technique in which in-plane uniformity of the gate width 9 is improved by controlling the surface temperature distribution of the holder 28 of the specimen 1 to control the in-plane temperature distribution of the specimen 1. Thus, this technique can improve the in-plane uniformity of the gate width 9. However, there is a demand for a further improvement in uniformity of the CD shift caused by variations in the surface density.
The following is a description of nonuniformity of the CD shift caused by variations in the surface density. The photoresist mask 6 is not always uniformly formed on the surface of the specimen 1. For example, components of the photoresist mask 6 are close to each other in some portion. In other words, the photoresist mask 6 is formed densely in some portion (hereinafter referred to as dense portion). In contrast, components of the photoresist mask 6 are far from each other in another portion. In other words, the photoresist mask 6 is formed sparsely in another portion (hereinafter referred to as nondense portion). The amount of polysilicon to be etched is smaller in the dense portion than in the nondense portion. Thus, in the dense portion, a smaller amount of reaction product containing silicon derived from the polysilicon film 4 is produced; therefore a smaller amount of reaction product is deposited on the sidewall of the gate electrode 7. Hence, even when the nondense portion has the same photoresist mask width 8 as the dense portion, the nondense portion may have a larger CD shift and a larger gate width 9 than the dense portion. However, when the distance between adjacent components of the photoresist mask 6 is very small in the dense portion, the amount of radical etchant entering the space between the adjacent components is smaller than that in the nondense portion. Thus, the dense portion may have a larger CD shift than the nondense portion. Furthermore, when a processing gas has a high deposition power, the amount of deposits on the sidewall of the gate electrode 7 resulting from the processing gas is smaller in the dense portion. Thus, the dense portion may have a smaller CD shift than the nondense portion. The variations in the surface density also reduce in-plane uniformity of the CD shift and should therefore be decreased.
In general, a specimen composed of a plurality of films needs a plurality of etching steps and nonoperating steps between the etching steps. In nonoperating steps, plasma discharge is stopped to eliminate a processing gas. This is because the etching conditions vary greatly with the film type. Thus, the target CD shift cannot be achieved by a continuous plasma discharge throughout the steps. However, the nonoperating steps decrease the throughput.